As an Accident and Emergency doctor, I see lots of people who have fallen off their bicycles, although the most seriously injured tend to get knocked off by motorists. "And was she wearing a cycle helmet?" is a standard question put to the parent of any injured child. Cycle helmets prevent injuries and save lives. That's what we are all told.

But there's always a niggling doubt in my mind, and those of many other doctors, that bike helmets aren't really all they are cracked up to be.

Why? Take these two examples: a six-year-old who slides off his bike on the driveway, and suffers nasty grazes and bumps to the head, entirely preventable had he been wearing a helmet; and a man of 40 flung from his bike into the path of a lorry by a passing car - he's dead from massive "non-survivable" head injuries. Even the best-designed bike helmet would have made no difference.

It's nice to think that between these two extremes there must be cases where helmets would make a significant difference to the incidence of long-term disability or death. Nice, but probably incorrect.

The BMA's board of education and science, which has examined the evidence, has just concluded that helmet use should not be compulsory in the UK. It is estimated that only around 18% of British cyclists wear them. In Australia, the home of helmet regulation, their use is mandatory. Deaths and serious head injuries among cyclists in Victoria fell by around 45% in the year following legislation in 1990. But so did the number of people riding bikes - by 40% in adults and 60% in children. Is this the real reason for the apparent drop in injuries and deaths?

Opponents of legislation point to New Zealand, where a sharp rise in voluntary helmet use in the months prior to a new law being enforced was not matched by any reduction in the rate of serious head injuries.

The Snell Memorial Institute in California was set up in memory of an amateur motor racer who died in 1956 when his "state-of-the-art" helmet failed completely to protect him. A Snell certification label is the gold standard in terms of safety certification of protective headgear (Snell B95 is the one to look for when buying a cycle helmet.) But the institute has never advocated any specific cycle helmet law.

Dr. George Snively, a founder, has said "it is impossible to build a [cycle] helmet that will offer significant impact protection".

A live brain is said to have the consistency of blancmange. Putting blancmange in a polystyrene box will not allow you safely to throw it against concrete without the contents being just as badly shaken as had the "protection" not been present.

"Bike helmet saved my life" makes a headline. But such claims often follow off-the-cuff comments by doctors like me. Few of us who attend people with cycle injuries are likely to be experts on the mechanics of impact injury. Hundreds of people fall from bikes every day while not wearing helmets, and avoid serious injury. These cases go unreported.

Campaigners for helmet laws approach the issue of cycle safety from the wrong direction, because two thirds of the most serious bike accidents are caused by car-drivers. This is certainly our experience in A&E departments, and something the BMA report also acknowledged. It calls for legislation aimed at drivers rather than cyclists; in particular, increased use of 20 mph speed limits in urban areas (although even at 20mph, and even if wearing a helmet, a direct impact to the head is likely to be fatal.)

Pedestrians and car occupants are in fact more likely to suffer head injuries from road accidents than cyclists. In the US, 34% of fatal head injuries happen to people in cars. Some 7% are pedestrians, and only 1% cycle riders. Yet no-one seriously suggests that helmets are worn by anyone other than cyclists and motorcyclists.

Cycling is good for you - as long as you don't get knocked off. Forcing people to wear helmets demonstrably reduces the number willing to ride a bicycle. Less cycle use means more obesity, heart attacks, and use of other, less environmentally-friendly means of transport. Making helmet use compulsory gives cycling an undeservedly dangerous profile, and may discourage bicycle use even further. Rather than encasing cyclists in armour-plating, we should be directing our attention to that nut behind the steering wheel.